Which Federal Aviation Regulation requires all electrical circuits to have some form of protective device?

The correct answer relates to the regulations set forth in 14 CFR Part 23, which specifically addresses airworthiness standards for normal category airplanes. This regulation includes requirements for the electrical systems of aircraft, mandating that all electrical circuits must be equipped with protective devices. Such devices are crucial for ensuring the safety and reliability of the electrical systems by preventing overloads and short circuits, which can lead to catastrophic failures.

In contrast, other parts of the regulations, while they may touch on aspects of aircraft maintenance and operation, do not specifically mandate protective devices for electrical circuits within an aircraft. For example, 14 CFR Part 91 primarily governs general operating and flight rules, Part 43 deals with maintenance, preventive maintenance, and alterations, while Part 25 focuses on the airworthiness standards necessary for transport category airplanes. These sections may address safety broadly but do not include the specific requirements for protective devices in electrical circuits as outlined in Part 23.